Replace all is_null($value) calls with null === $value
or alternativly null !== $value
for consistency in the framework
added braces to null comparison
Merge branch 'null' of git://github.com/prolic/zf2 into hotfix/3520
[#3520] Remove yoda conditions in null checks in DB

- per @ralphschindler
Merge branch 'hotfix/3520'

Close #3520